1. Field of the Invention
The present invention relates to a mobile communication system and a method for determining a base station antenna proximity state.
2. Description of the Related Art
In the mobile communication system using a wireless medium as a communication medium between a mobile terminal and a base station, improving the receiving efficiency of a signal is effective to increase the communication capacity. Higher receiving efficiency can lower transmission power for meeting the same signal quality, which can reduce interference power caused by transmission power generated from the terminal or the base station. Generally, the received signal quality such as a bit error rate is determined by the ratio of desired signal power to interference power. For improving the quality of the received signal, the desired signal power needs to be increased or the interference power needs to be decreased.
In order to enable as many mobile terminals as possible to perform communication in the case where mobile terminals are present in an area covered by a plurality of wireless base stations, the base station having the highest receiving efficiency and the mobile terminal need to communicate with each other. That means that a mobile terminal communicates with the closest base station in terms of propagation space. The shortest distance in terms of propagation space is the closest geographical distance on flat land. In urban areas having clusters of buildings, however, the buildings or the like reflect or scatter a wireless signal, thus, a distance in terms of propagation space is not necessarily short for even if the geographical distance is close. Therefore, a mobile terminal device estimates and determines a base station to place a communication link by estimating the distance between the mobile terminal and a base station in terms propagation space. In the estimation, power transmitted from a base station received at the mobile terminal is measured and its propagation loss is estimated by an attenuation of the power.
As another method for improving the receiving efficiency, a space diversity technique can be used. The technique includes a receiving diversity for receiving a radio wave transmitted from a single transmission point at a plurality of different positions in terms of propagation space and a transmitting diversity for transmitting a radio wave from a plurality of different positions in terms of propagation space to a single receiving point. In an actual mobile communication system, a receiving gain can be improved as a radio wave transmitted from a single mobile terminal is received at a plurality of base station antennas and the signals received at respective antennas are combined. The improved receiving gain can reduce transmission power for maintaining a predetermined signal quality. Accordingly, the mobile terminal's interference power that affects another mobile terminal decreases so that communication capability from the mobile terminal to the base station antenna can be increased.
The Code Division Multiple Access system includes methods for increasing the receiving efficiency by receiving a signal transmitted from a single transmitting point at a plurality of different receiving points. The first method one is a selection combining for decoding a signal transmitted from a single transmitting point into base band signals at a plurality of receiving points respectively, performing quality evaluation on the signal received at each point, determining whether the signal is valid (without an error) or invalid (with an error), and determining the signal to capture. The second method is a maximum ratio combining for aligning signals received at a plurality of receiving points such that they arrive at the same time and adding the signals, then decoding the signal into the base band signal.
In the selection combining method, any one of the signals received at the plurality of receiving points needs to be valid (without an error). If two or more signals are valid, only one of them is to be referenced. Many invalid signals each having a few errors do not work at all.
The maximum ratio combining method has a feature of adding energies of respective signals received at a plurality of receiving points. Thus, if the signals received at the received points are collected, receiving power that is sufficient for decoding a signal can be obtained, even if the receiving power received at only a single receiving point is insufficient. As a result that makes the receiving efficiency higher than in the case where a signal is received at a single point. Thus, this can reduce the transmission power at a transmission point required for providing a predetermined signal quality.
The selection combining reception and the maximum ratio combining reception methods differ in their required transmission channels. In the selection combining reception method, because the transmitted signal is decoded into the base band signal at the receiving points, the signals only need to be collected at a combining point through transmission channels having a similar information rate as that for the base band signal. Thus, a low speed transmission channel is sufficient for the transmission channel between the receiving point and the combining point. In contrast, in the maximum ratio combining method, the transmitted signal is not decoded into the base band signal at the receiving points and the signals in a wireless section base need to be collected because they are at a combining point through transmission channels. Thus, a high speed transmission channel is required for the transmission channel between the receiving point and the combining point. Therefore, the actual maximum ratio combining is only applied in the case where the receiving point and the combining point are geographically close so that a high speed transmission channel can be ensured between the receiving point and the combining point. In the other cases, the selection combining is adopted. That is to say, the maximum ratio combining is a combining method that is performed between sectors of sector base stations in which a single base station has a plurality of receiving antennas. The selection combining is a combining method that is performed between base stations that are geographically apart from each other.
As mentioned above, whether the selection combining or the maximum ratio combining methods are applied is logically determined by whether the positions of the plurality of receiving points are geographically close or not. In order to improve the receiving efficiency, however, it is desirable to use the maximum ratio combining reception method whether the receiving points are close or not.
In order to use the maximum ratio combining method, a high speed transmission channel for transferring signals from a plurality of receiving points by signals in the wireless section base is necessary. In order to effectively perform maximum ratio combining reception in a mobile communication system in which a plurality of antennas form a service area, the receiving points whose covering areas are close need to be connected by high speed transmission channels in general terms. Strictly speaking, the maximum ratio combining reception can be effectively performed because signals from a plurality of receiving points that can be received with a certain degree of receiving power are collected to a combining point by a high speed transmission channel.
As mentioned above, in order to use the maximum ratio combining reception, signals in wireless section base received at a plurality of receiving points need to be combined. In such a case, maximum ratio combining is generally performed by having a plurality of antennas, that have different directivity, set in the same base station that is used as a receiving point. This is why it is easy to ensure a high speed transmission channel if the antennas are in the same base station. The maximum ratio combining, however, needs to ensure a high speed transmission channel for a long section in which points are geographically apart from each other. Under the circumstance of a low information transmission rate of the wired transmission channel, it has been considered that the maximum ratio combining is not very realistic. The maximum ratio combining has a feature of improving the receiving efficiency because a signal transmitted from a single point can be received at a plurality of antennas and the signals can be added in terms of energy in the case where a radio wave transmitted from a single mobile terminal is received at a plurality of antennas. Therefore, performing the maximum ratio combining not only between receiving points such as antennas that are contained in a single base station but also between geographically different receiving points as mentioned above may provide an effect of further improving the receiving efficiency.
Connecting any receiving points geographically close to each other by a high speed transmission channel, however, causes large amount of consumption of transmission band in a backbone circuit, and thus increases the cost for establishing a high speed transmission channel. In that case, it is not ensured that all the connected high speed transmission channels are used, resulting in wasted high speed transmission channel. Therefore, in terms of an efficient and pragmatic solution, there is a need to find a pair of receiving points, from among receiving points that are geographically close to one another, that will be effective for performing maximum ratio combining. Thus, a method for finding a base station that a mobile terminal placed in a service area can communicate with as a mobile terminal that is not in communication estimates strength of a downlink electric field from each of the base stations around the mobile terminal and selects a base station with a high strength of the received signal is adopted. A method to cause a mobile terminal that is in operation to select a base station with which it can communicate when the mobile terminal moves by notifying the terminal of the ID of a candidate base station, and that causes the terminal to measure the strength of a downlink electric field from base stations and to inform the strength to the base stations and to select an appropriate base station based on the result, is performed in a handover procedure. For information on surrounding base stations used here, it is common to use a method for placing a particular receiving device in a certain base station, causing the receiving device to measure the strength of the downlink electric field from the plurality of base stations and to generate the information on the surrounding base station based on the measured result. The method is disclosed in Japanese Patent Laid-Open No. 2002-84567, for example.
Such a receiving device that measures the strength of a downlink electric field from surrounding base stations, however, is often created by an object transported on a vehicle that is equipped with a storage device featuring large capacity and higher receiving performance, and the range for such a receiving device is limited to a channel on which the vehicle can move. The height of antennas of devices for measuring the strength of an electric field significantly differ from the height of an antenna of a terminal that is actually used by a user. Such a receiving device has a problem in which it is difficult to make measurements that reflect the changing locations of a mobile terminal actually used by a user in a service area, and thus it is difficult to correctly determine the base station antenna with which the mobile terminal communicates.